Analysis of cast tool steel of 55NiCrMoV7 type cast in precision ceramic mould.

Beznak, Matej ; Chaus, Alexander ; Bajcicak, Martin 等

1. INTRODUCTION

In the past castings, as a rule, had relatively large allowances for processing due to bad quality of their surface. Today the modern trend is to produce castings with the final dimensions and good surface quality. One of the methods, which provide needed accuracy and quality of castings, is producing composite ceramic moulds in accordance with Shaw process. The application of Shaw process makes it possible to produce castings from all known foundry alloys with no presently known size restriction that can be the main advantage of this technology. The very castings produced in ceramic moulds by Shaw process have high degree of size precision and outstanding metal soundness in term of density as well as surface quality, that benefits in improvements of both energy and materials savings, and processing time reduction. In this term importance of Shaw process increases dramatically when casting expensive tool steels, high alloy steels, and special alloys, which are the difficult-to-machine materials (Beznak, 2003).

2. SHAW PROCESS IN PRODUCTION OF CERAMIC MOULDS

For casting die inserts the composite ceramic mould, which is comprised of a backing layer and a facing layer, is produced as shown in Fig. 1. Pouring a mixture of a chamotte refractory and a water glass binder about an oversized pattern forms the backing layer. After the backing layer hardens, the facing layer is then formed integrally with the backing layer by pouring a refractory slurry-like mixture between the oversized backing layer and a dimensionally correct pattern. When this mixture forms a somewhat flexible gel, the mould can be stripped off the pattern. After it facing layer is fired and then baked (Beznak, 2004).

[FIGURE 1 OMITTED]

In general, mixture for producing facing layer of a ceramic mould is composed of a highly refractory material, a gelling agent, and a binder. As a refractory material may be used sillimanit, mullite, zircon. We used two types of the refractory materials, which differ in particle size. The first refractory material was of a finer grade with the particle size less than 0.5 mm. The second refractory material was of a coarser particle size in the range from 1 till 3 mm. The slurry-like mixture hardens when gelling agent, which is 15 % sodium hydroxide solution, is added in amount of 10-12 g per 1 kg of the mixture to ethyl silicate binder. Due to hydrolysis of ethyl silicate, induced by NaOH, silica gel is formed. Rate of the gel formation or gelling rate depends on PH of a sodium hydroxide solution.

When mixing all components, the slurry-like mixture is formed and its PH is about 10 just before pouring. For castings of common sizes gelling time of the mixture is 30--180 seconds. This time depends on the both pattern sizes and amount of the slurry-like mixture used (Beznak, 2005).

The slurry-like mixture has very good contact with the pattern that results in lower surface roughness of castings. Due to high elasticity of the mould served in the first stage it can be easy stripped off the pattern without risk of destruction.

In the Shaw process, after setting and stripping the pattern, the mould is immediately subjected to a rapid, uniform and intense flame firing with the aim to remove the alcohol generated during hydrolysis. By flame firing water is also removed from the mould. Dehydratation is accompanied by increase in amount of the retained SiO2 that results in higher hardness and lower plasticity of the ceramic mould material. Last but not list, when the moulds are fired the rapid burning and intense heat cause micro-cracks to develop, which renders a dimensional freezing, so that the moulds are immune to subsequent severe thermal shocks. Beside this micro-cracks provide better gas permeability of the ceramic mould.

After flame firing ceramic moulds are subjected to a baking at 1000[degrees]C. After the baking, moulds are assembled and then cooled for 24 hours. After it the moulds are prepared for pouring.

Fig. 2a shows the composite ceramic mould produced for die insert casting and comprised of the facing layer and the chamotte-basis backing layer. Fig. 2b shows the die insert cast in the composite ceramic mould produced in accordance with Shaw process.

[FIGURE 2 OMITTED]

3. STUDY OF THE TOOL STEEL AFTER CASTING AND HEAT TREATMENT

3.1 Experimental Procedure

Chemical composition of cast tool steel of 55NiCrMoV7 type used in this study is presented in Table 1. The steel was melted in an electric middle-frequency induction furnace and after additional alloying and deoxidising was poured into ceramic moulds shown in Fig. 2a. Four specimens from this steel for different structural states (after casting, annealing, quenching, and tempering) were prepared from the cast die insert shown in Fig. 2b. The specimen's sizes were 7x7x17 mm.

The experimental castings and specimens were isothermally annealed at 880[degrees]C for no less than 2 h. The cooling down to 500[degrees]C was performed in a furnace and, then, in air. To prevent decarburisation, when annealing, the castings and specimens were covered with crashed cast-iron chips. The final heat treatment of the materials included quenching and tempering. When quenching, the specimens were cooled from the temperature of 850[degrees]C in oil. The specimens were held at austenitising temperature in a molten salt (44% NaCl + 56% KCl) for 1 min per 1 mm of the specimen cross section. Cooling of the specimens during quenching was carried out in the oil. The specimens were tempered at 550[degrees]C for 2 h in a molten salt KNO3.

3.2 Metalographic analysis of the cast tool steel studied

Fig. 3 shows microstructure of the steel of 55NiCrMoV7 type after casting.

Microstructure is composed of fine ferritic and perlitic constituents without defects like non-metallic inclusions, gas and shrinkage porosity. In some local areas the martensite as well as the primary carbides can be seen as shown in Fig. 3b. According to the typical shape of these carbides they can be classified as MC-type carbides, namely vanadium-reach VC carbides. The microhardness of these carbides, which is 1800-1850 HV0.1, is in line with such prediction. The VC carbides seem to be formed due to strong segregation of vanadium in the remaining melt during primary solidification of the steel.

Fig. 4a shows the structure of 55NiCrMoV7 tool steel after annealing. It is seen from this figure that the steel matrix has typical dendritic structure after annealing that inherited from cast state.

Martensitic and troostitic structure is typical for this tool steel after quenching that shows Fig. 4b. The structure is formed in the frame of the primary dendrites, which have the evident steps of segregation of alloying elements that leads to the different level of etching of the dendrites in their cross sections as shown in Fig. 4b.

[FIGURE 3 OMITTED]

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[FIGURE 5 OMITTED]

Fig. 5a shows that the structure of the steel after tempering is composed of fine troostite. Despite this the structural heterogeneity is also seen in Fig. 5b that resulted from the presence of the random primary carbides of VC type in the tempered structure.

4. CONCLUSION

Solidification sequence of 55NiCrMoV7 tool steel cast in the ceramic mould provides formation of the fine solidified structure, which is composed of the fine ferrite and perlite without defects like non-metallic inclusions, gas and shrinkage porosity. Heat treatment does not change the typical signs of cast structure in general but leads to formation of the fine troostitic structure after tempering. Such structure seemed to be suitable in terms of mechanical and working properties of cast die inserts cast in the ceramic moulds in accordance with Shaw process (Beznak, 2007/a, b). Employment of the tool steel specially designed for cast die tools is very effective from viewpoint of the impact toughness and durability of the cast die tools.

5. ACKNOWLEDGMENT

The financial support of grants from the Ministry of Education of the Slovak Republic VEGA 1/4109/07 and VEGA 1/3191/06 is gratefully acknowledged.

6. REFERENCES

Beznak, M. (2007/a). Production of the precision castings in ceramic moulds using permanent pattern. International science conference of materials science and manufacturing technology, 157-161, ISBN 978-80-213-1650-8, Prague, 2007

Beznak, M. (2007/b). Production of die tools by precision casting into ceramic moulds. Alumni press, ISBN 978-80 VM/2007, Trnava

Beznak, M. (2005). Production of forging tools by precision casting into ceramic moulds. Masinostroenie. Vol. 21, No. 1, (2005) 281-285 ISBN 985-479-322-2, Minsk

Beznak, M. (2004). Possibilities of forging die inserts production by precision casting using permanent pattern. Cooperation international conference. (10,2004), 97-102

Beznak, M. (2003). Main aspects influencing of the technology of precision ceramic moulds. CO-MAT-TECH2003, 60-63, ISBN 80-227-1949-8, Bratislava

Table 1. Chemical composition of cast tool steel of

Steel Chemical composition (wt.%)

 C Si Mn P S

55NiCrMoV7 0.39 1.03 0.35 0.02 0.016

 Chemical composition (wt.%)

 Cr Mo V Ni Ti

55NiCrMoV7 5.22 1.32 1.10 1.12 0.04

 Chemical composition (wt.%)

 Nb N Al

55NiCrMoV7 0.03 0.02 0.014